Steel Defender of Battle Smith and how to interpret spell attack modifier as to hit bonus

My battle smith artificer has an attuned all purpose tool which boosts his spell attack modifier. On DnD Beyond, the to hit bonus of my steel defender is the initial modifier, not the boosted amount. To be specific, the default modifer is +7, but rare all-purpose tool boosts it to +9 after attunement. For steel defender in the actions tab, it is still +7.

For fellow players and DMs, should I use the boosted spell attack modifier or default modifier? 

It says that its attack bonus "equals your spell attack modifier", so if your spell attack modifier increases, its attack bonus increases. For it to work the other way it would need to say "equals your spell attack modifier at the time you gain this feature" or something like that, but it doesn't. So, in your example the Steel Defender's attack bonus is +9 while you're attuned to that item; if your attunement ended for whatever reason, it would drop back to +7.

The fact that the feature on D&D Beyond's character sheet says +7 is a bug in D&D Beyond's implementation of the rules; it doesn't affect what the actual rules say.

Thanks! I will mention this to my DM at my next game and will manually add the bonus to that specific action. 

Sounds similar to dndbeyond not calculating True Strike bonuses properly.

Quote from MongooseGef >>

Sounds similar to dndbeyond not calculating True Strike bonuses properly.

Yeah, the character sheet and character builder have a lot of limitations due to having been designed around what was the current state of the D&D rules back when it was first built (pretty early in the 5e era) and not factoring in much or any capacity for easily extending the design to accommodate newer stuff.